1. Field of the Invention
This invention relates to a semiconductor switch which has a four-layered structure of a P-type emitter, an N-type base, a P-type base and an N-type emitter and which has at least three PN-junctions.
Hereinbelow, the semiconductor switch having the PNPN structure shall be simply termed "PNPN switch."
The PNPN switch has the disadvantage that, even in the cut-off state, the switch is improperly ignited when an abrupt forward voltage is applied between the P-type emitter (anode terminal) and the N-type emitter thereof. Such a phenomenon is called the "dv/dt effect" or "rate effect." In order to avoid the phenomenon, some proposals have heretofore been made. The withstand strength up to which the PNPN switch is not improperly ignited by a transient voltage (dv/dt) is generally termed the "dv/dt bearing capacity."
2. Description of the Prior Art
As proposals for enhancing the dv/dt bearing capacity, U.S. Pat. No. 3,609,413 and U.S. Pat. No. 4,015,143 have been known. In both the proposals, a transistor is connected between the P-type base and N-type emitter of a PNPN switch, and only when an abrupt forward voltage is applied between the P-type emitter and N-type emitter of the PNPN switch, the transistor is turned "on" to electrically short-circuit the path between the P-type base and the N-type emitter, whereby any improper ignition is prevented.
According to these methods, certainly the dv/dt bearing capacity during the period during which the abrupt transient voltage is applied is enhanced. In some cases, however, an erroneous or improper ignition occurs after termination of the transient voltage.
This phenomenon is such that, when the short-circuiting transistor turns "off" at the time of the termination of the transient voltage, the voltage between the P-type base and N-type emitter of the PNPN switch becomes higher than the activating voltage of the PNPN switch, with the result that the switch is improperly ignited.
A factor for the occurrence of the improper ignition is the recovery time constant of the PNPN switch. When the recovery time constant is great, the erroneous ignition is prone to take place.
The recovery time constant is determined by the integration structure of the PNPN switches. In general, it increases as the density of integration is raised. Accordingly, it has heretofore been limited to enhance the dv/dt bearing capacity and to simultaneously raise the density of integration.